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doi:10.2204/iodp.proc.330.107.2012

Physical properties

Characterization of physical properties was conducted for rocks recovered from Hole U1376A through measurements on whole-round and split-core sections and discrete samples. Measurements of gamma ray attenuation (GRA) bulk density, whole-round and point magnetic susceptibility, laser height, and color reflectance were conducted on all 121 core sections recovered from this hole. Whole-round core sections longer than ~50 cm (117 of 121 available sections) were also run through the Natural Gamma Radiation Logger (NGRL). Discrete measurements included compressional wave (P-wave) velocity and moisture and density measurements on 71 discrete oriented rock cubes. Most of these discrete samples were also used for paleomagnetic measurements of alternating-field demagnetization (see “Paleomagnetism”). No thermal conductivity measurements were made because of equipment failure earlier in the expedition. In accordance with core depth below seafloor Method A (CSF-A) conventions to reference cores to depth (see “Procedures” in the “Methods” chapter [Expedition 330 Scientists, 2012a]), data from cores with >100% recovery (e.g., Cores 330-U1376A-12R and 13R) are shown in figures as overlapping. In general, all physical property data sets are mutually consistent and show distinctions and trends often correlating with lithologic changes and stratigraphic unit boundaries (see “Igneous petrology and volcanology”) and with petrologically determined alteration trends (see “Alteration petrology”).

Whole-Round Multisensor Logger measurements

Throughout the lithified sediments and igneous basement of Site U1376, individual sections generally contain multiple discrete pieces, as is typical of hard rock coring. In order to remove spurious Whole-Round Multisensor Logger (WRMSL) and Section Half Multisensor Logger (SHMSL) data affected by the gaps and edge effects from these discontinuities, we applied a data filtering and processing algorithm (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). In this report we show only the filtered data; for raw data we refer the reader to the visual core descriptions (see “Core descriptions”) and the Laboratory Information Management System (LIMS) database (iodp.tamu.edu/tasapps/).

Magnetic susceptibility

Magnetic susceptibility is sensitive to the mineralogical composition of the rock. Whole-round magnetic susceptibility measurements are shown in Figure F42. The depth profile for magnetic susceptibility at this site consists of distinct short-wavelength peaks with values that exceed 1.00 × 10^–2 SI, as well as broad regions of consistently lower values.

Stratigraphic Unit I, composed of lithified volcanic sediments, has a relatively low average magnetic susceptibility of 1.31 × 10^–3 SI. However, the unit begins and terminates with comparatively high values, whereas the middle portion is characterized by very low consistent magnetic susceptibility measurements. This pattern likely reflects the higher proportion of relatively fresh basalt clasts near the unit boundaries. The algal limestone of Subunit IIA is marked by very low magnetic susceptibility that increases slightly toward the base of the unit. This pattern agrees well with the observation that the upper portion is composed mostly of carbonate material, whereas the lower portion has increasing amounts of volcanic sediments incorporated into the structure (see “Sedimentology”). In the conglomerate, highs in magnetic susceptibility are associated with both larger basaltic clasts and black sandy intervals. In the upper portion of Unit III, the marked highs are associated with intervals dominated by hyaloclastites, rather than with lava lobes, flows, or pillows.

A marked increase in magnetic susceptibility occurs at 72 mbsf and remains relatively consistent down to 105 mbsf. This interval has an average magnetic susceptibility of 4.16 × 10^–3 SI and coincides with the identification of a 33 m thick lava flow (see “Igneous petrology and volcanology”). A slight downhole increase in average magnetic susceptibility from 3.89 × 10^–3 SI to 4.53 × 10^–3 SI was observed in this interval. In contrast, the volcanic breccia from the upper portion of stratigraphic Unit IV yields consistently low magnetic susceptibility values (average = 1.27 × 10^–3 SI), whereas the breccia found below 135 mbsf is characterized by much higher values (average = 4.66 × 10^–3 SI) and significantly more scatter. This change likely represents the transition from hyaloclastite-dominated breccia to a multilithic volcanic breccia containing both aphyric and olivine-phyric basalt clasts. The dikes represented by lithologic Units 20 and 41 have a magnetic susceptibility close to that of the thick lava flow, with an average of 3.92 × 10^–3 SI.

Gamma ray attenuation bulk density

The results of GRA-derived bulk density are shown in Figure F43. A correction factor of 1.138 was applied to account for the smaller average diameter (58 mm) of hard rock cores compared to the full core liner diameter of 66 mm (see “Physical properties” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). Values of <1.00 g/cm³ were attributed to empty portions of core liner and removed. Bulk density at this site ranges from 1.17 to 3.19 g/cm³, with an average of 2.50 g/cm³.

Although a significant amount of scatter in GRA-derived bulk density values was observed for much of the core, as is typical of volcanic breccia, a number of trends were identified in the stratigraphic and lithologic units. The lithified volcanic sediments of stratigraphic Unit I have a relatively low average bulk density of 2.00 g/cm³ and demonstrate a downhole-decreasing trend. This trend terminates at the beginning of Subunit IIA, at which point a slight downhole-increasing trend begins for the carbonate material, which averages 2.43 g/cm³. The highest observed bulk density was found in the interval from 72 to 105 mbsf, which correlates with a thick olivine-phyric lava flow. GRA-derived bulk density measurements in this interval have remarkably little scatter, with an average of 2.96 g/cm³. A slight, steady increase with depth from 2.93 to 3.02 g/cm³ was also discernible for this flow, and this trend may correlate with a subtle decrease in porosity or an increase in the percentage of olivine phenocrysts. Other more localized increases in GRA-derived bulk density throughout the core can generally be attributed to lava flow lobes and dikes in the volcanic breccia, which has a background average of 2.44 g/cm³. A localized decrease in density was also observed from 123.5 to 126 mbsf, corresponding to an interval of hyaloclastite with very few basaltic clasts.

Natural Gamma Radiation Logger

Natural gamma radiation (NGR) measurements reflect the combined total amount of uranium, thorium, and potassium present in the rock. Results from the NGRL are shown in Figure F44. NGR ranges from 0.49 to 26.42 counts per second (cps), with an average value of 12.42 cps.

Although variable, many of the highest NGR values measured at this site occur in the volcanic sandstone of Unit I. The algal limestone of Subunit IIA exhibits the lowest levels of NGR measured during the expedition, with an average of 1.64 cps. NGR counts for the base of this carbonate interval are as high as 12.32 cps near the lowermost recovered portion of Subunit IIA and then sharply decrease uphole to a site minimum of 0.49 cps. This decrease likely reflects the transition from a volcanically influenced depositional environment to a clastic-free shallow-marine environment (see “Sedimentology”). The upper portion of stratigraphic Unit III displays moderate variability in NGR, reflecting both lithologic and alteration changes. The peaks at 42 and 46 mbsf occur in the breccia and correlate with increased alteration, whereas those at 52 and 55 mbsf correlate with lava flow lobes.

Below these localized peaks, the interval from 60 to 130 mbsf is characterized by an essentially featureless band of NGR between 9 and 14 cps, with an average of 12.06 cps, interrupted by a single peak associated with the dike of lithologic Unit 20 (recovered in the uppermost part of stratigraphic Unit IV). Unlike magnetic susceptibility and GRA-derived bulk density measurements, NGR results show no distinctive signature associated with the 33 m thick massive lava flow of Unit III. The volcanic breccia below 130 mbsf has a lower overall average of 9.26 cps but demonstrates a downhole-increasing trend, culminating with moderate values of 10–20 cps, representing the lowermost dike (lithologic Unit 41). Short-wavelength peaks in this interval correlate well with the intercalated lava flow lobes.

Section Half Multisensor Logger measurements

Color reflectance spectrometry

Color reflectance spectrometry results are summarized in Figure F45. L* (lightness) of the recovered core has a median value of 33.4. The algal limestone composing Subunit IIA has markedly higher L* than the rest of the hole (median value = 71.1), reflecting the bright white nature of this carbonate lithology. Although it remains very high throughout the algal limestone of Subunit IIA, L* also exhibits a downhole-decreasing trend that correlates with the increasing percentage of volcanic grains in this limestone from top to bottom (see “Sedimentology”). The other sedimentary units (stratigraphic Unit I and Subunit IIB) also exhibit higher L* values than the site average, but not nearly to the degree of the algal limestone. L* is relatively uniform in magnitude and variance in most of the igneous basement but is slightly reduced in both magnitude and variance for the interval from ~112 to 123 mbsf. This region of Unit IV is composed of a higher proportion of hyaloclastites containing a high proportion of fresh volcanic glass (see “Igneous petrology and volcanology”). Also, the median absolute deviation of L* in the olivine-phyric breccia below 130 mbsf is significantly higher (10.2) than that for both the olivine-augite-phyric breccia and olivine-phyric breccia above 130 mbsf (6.7 and 7.4, respectively). This indicates a statistically higher amount of scatter in L* in the lower portion of Unit IV than in the rest of the igneous material, possibly reflecting the multilithic nature of this breccia.

Figure F45 also shows values of a* and b*, which correspond to redness versus greenness and yellowness versus blueness, respectively. The sedimentary rocks of Unit I are characterized by strongly red and yellow spectra, with an especially high amount of scatter in the b* values representing variations in the degree of yellowness. The amount of scatter likely reflects the range of clast types in these volcanic sediments. The overall redness and yellowness of this unit may be indicative of an oxidizing environment during deposition. The color reflectance values of the algal limestone of Subunit IIA also correspond to predominantly red and yellow spectra, with local variations possibly attributed to a slight increase in iron oxides compared to the surrounding limestone.

Igneous stratigraphic Units III and IV have a predominantly green to red/green neutral spectrum consistent with the predominantly green alteration (see “Alteration petrology”). The a* and b* values from igneous Units III and IV also demonstrate a distinct inverse relationship, such that when a* transitions to a more green spectrum (more negative), b* transitions to a more yellow spectrum (more positive), and vice versa. The significance of this pattern, which was not identified at any of the previous Expedition 330 sites, is unclear. The volcanic breccia exhibits a predominantly green and yellow spectrum to 127 mbsf, below which no clear relationship between lithologic units and color reflectance can be discerned. This observation may reflect a transition from breccia containing olivine-phyric clasts to multilithic breccia containing both aphyric and olivine-phyric basalt clasts. The addition of blue-gray aphyric basalt clasts may shift the otherwise green and yellow spectrum of the breccia to neutral a* and negative (blue) b* values. The thick lava flow from 72 to 105 mbsf is characterized by a moderate reduction in the short-wavelength scatter of a* and b* but is not otherwise delimited by color reflectance data, showing several subtle variations between a red/green neutral and blue spectrum and a green and yellow one.

Point magnetic susceptibility

Point magnetic susceptibility results are shown in Figure F42 together with whole-round magnetic susceptibility data. The pattern of peaks and troughs in this data set agrees well with the whole-round data, but the point magnetic susceptibility results are consistently lower than the whole-round results. The average values for point magnetic susceptibility are 1.01 × 10^–3 SI for the volcanic sandstone, 3.41 × 10^–4 SI for the carbonates, 2.92 × 10^–3 SI for the thick olivine-phyric lava flow, and 2.22 × 10^–3 SI for the volcanic breccia.

Moisture and density

Bulk density, dry density, grain density, void ratio, water content, and porosity measurements on discrete samples are listed in Table T10. Bulk density ranges from 1.75 to 3.10 g/cm³, with an average of 2.54 g/cm³. Porosity ranges from 1.2% to 57.9%, with an average of 18.2%. As illustrated in Figure F46, a strongly linear negative correlation between bulk density and porosity was observed. Bulk density measurements from discrete samples also agree well with GRA-derived bulk density measurements, as shown by Figure F47. The near one-to-one linear relationship between these two data sets supports our 1.138 volume correction factor for GRA-derived bulk density. GRA-derived bulk density values may be affected by the presence of fractures and cracks in the whole-round cores, slight variations in core radius (approximately ±1–2 mm), and distortions of the core’s cylindrical shape near piece ends or from large voids. These factors can all cause overestimates of the total volume used in the GRA-derived bulk density calculations even after the correction factor is applied, thus explaining why some GRA-derived bulk densities remain slightly lower than the corresponding results from discrete samples.

Figure F43 shows the variation of bulk density with depth based on both discrete samples and GRA-derived bulk density and further illustrates the strong correlation between the two. In the sedimentary units, a sharp transition is observed between the volcanic sandstone of Unit I, which has an average  bulk density value of 2.08 g/cm³, and the algal limestone of Subunit IIA, which has an average of 2.62 g/cm³, as determined from discrete sample measurements. Bulk density is consistently high throughout the massive 33 m thick lava flow within stratigraphic Unit III (lithologic Unit 15), with an average of 3.03 g/cm³, and also demonstrates a slight downhole increase, as previously noted for GRA-derived bulk density. The highly variable values obtained for measurements on discrete samples from the volcanic breccia intervals further illustrate their heterogeneous nature.

Figure F48 shows the variation of porosity with depth. The volcanic sandstone of Unit I is characterized by high porosity, averaging 38.4%. In Subunit IIA, the algal limestone has low porosity, with an average of 4.8%. These samples also deviate from the general linear trend between density and porosity (Fig. F46), illustrating their distinct chemical composition. In the volcanic basement, porosity in the volcanic breccia is highly variable, reflecting the contrasts between hyaloclastite matrix and basaltic clasts and flow lobes. Within the 33 m thick massive flow in stratigraphic Unit III, porosity is consistently very low, averaging 1.9%, and decreases slightly with depth. This downhole decrease in porosity explains part of the increase in bulk density. However, the grain density increases with depth, albeit with more scatter, suggesting that an increase in olivine may still contribute to the downhole increase in density. Five samples from the volcanic breccia in Cores 330-U1376A-14R through 16R show distinctly low density, lower than the GRA-derived bulk density (Fig. F43), and disproportionately low porosity relative to their density (Fig. F46). These samples are from an interval containing increased amounts of comparatively fresh hyaloclastite, so this unique signature could possibly be attributed to smectite infilling of vesicles or to sealed void spaces within the hyaloclastite that are not included in the porosity measurement.

Compressional wave (P-wave) velocity

The measured P-wave velocity of discrete samples shows a strong linear relationship with bulk density (Fig. F49). Downhole variations in P-wave velocity are shown in Figure F43 and Table T11. P-wave velocities are widely scattered in general because much of the material is breccia with intermittent lava flows, leading to large variability over even short depth intervals. P-wave velocities in Hole U1376A range from 1.78 to 7.21 km/s, with an average of 4.65 km/s. The lowest compressional wave velocities were measured in the volcanic sandstone of Unit I (average = 2.97 km/s). The algal limestone of Subunit IIA, by contrast, has a significantly higher average velocity of 6.19 km/s. These samples also form a distinct population in Figure F49, reflecting their distinct chemical composition. In the volcanic basement, many of the highest values can be attributed to lava flow lobes in basalt breccia, particularly the thick olivine-phyric basalt flow from ~72 to 105 mbsf. This interval is marked by highly consistent P-wave velocity measurements averaging 6.84 km/s. In the volcanic breccia itself, measurement of relatively high or low P-wave velocity depends on whether the discrete sample was taken from a large clast or the surrounding groundmass, respectively. Most samples show no statistically significant anisotropy; of those that do, the anisotropy has no consistent relationship with depth or lithology.

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